The European Telecommunications Standardisation Institute (ETSI) is currently in the process of standardising a new set of protocols for mobile telecommunications systems. The set of protocols is known collectively as the Universal Mobile Telecommunications System (UMTS). FIG. 1 illustrates schematically a UMTS network 1 which comprises a core network 2 and a UMTS Terrestrial Radio Access Network (UTRAN) 3. The UTRAN 3 comprises a number of Radio Network Controllers (RNCs) 4, each of which is coupled to a set of neighbouring Base Stations (BSs) 5—BSs are often referred to as Node Bs. Each BSs 5 is responsible for communicating with mobile terminals (or User Equipment (UE)) 6 within a given geographical cell, and the controlling RNC 4 is responsible for routing user and signalling data between a BS 5 and the core network 2. The interface between the RNCs is referred to as the Iur interface, whilst that between the BSs and the RNCs is referred to as the Iub interface. The air interface between the UE and the BSs is referred to as the Uu interface. FIG. 1 also illustrates the interfaces present in the conventional UTRAN. A general outline of the UTRAN 3 is given in Technical Specification TS 25.401 V3.3.0 (1999-09) of the 3rd Generation Partnership Project, 3GPP.
Under the UTRAN proposals, certain transmission channels (e.g. DCHs and DSCHs) make use of a mechanism known as Automatic Repetition reQuest (ARQ) to facilitate the retransmission of data packets which are either not received, or are received erroneously by a receiving entity, i.e. a UE or a RNC. The sending of an ARQ status message from a receiver may be ordered under this mechanism, the ARQ status message containing one or more ACKs, indicating the successful receipt of a packet, and/or one or more NACKs, indicating that a packet has not been received or has been received incorrectly. It will be appreciated that the retransmission path, involving as it does passage through a BS, can introduce a considerable delay into the retransmission time and can impact significantly on the performance of higher layer protocols (e.g. TCP).
One way to reduce the length of the retransmission path is to implement a new RAN architecture in which the ARQ mechanism is implemented in the BSs and not in the RNCs. FIG. 2 illustrates an architecture in which user plane and the time critical part of control plane handling is carried out in the BSs and the Gateway (GW) to the core network, whilst non-time critical control plane handling is carried out in the RNC. A new interface between the BSs is created, this being referred to hereinafter as the Iur+ interface. The interfaces of the new network are also illustrated in FIG. 2.
The conventional RAN architecture of FIG. 1 employs a mechanism known as soft handover to enhance system performance at cell borders and to facilitate the smooth transfer of UEs between BSs. During a soft handover, data may be sent and received in parallel between two or more BSs and a UE. At the receiver, information is combined at the physical layer before it is passed to the ARQ protocol (at layer 2). As ARQ is handled at the RNC, the retransmission of data causes no problems (other that the problem of retransmission delay already noted). For data transmission in the downlink direction, the RNC collects ARQ status messages returned from a UE (and forwarded by the various BSs involved in the soft handover) and sorts these to determine which data has been correctly received by the UE and which, if any, must be retransmitted. For data transmission in the uplink direction, the RNC is again able to co-ordinate the receipt of data packets sent from the UE and to transmit appropriate ARQ status messages to the UE.
For certain types of downlink channel it is not desirable to transmit data in parallel to a UE from several BSs. However, the soft handover process described may occur for uplink channels, with the (downlink) sending buffers of the soft handover BS set being synchronised in readiness for a hard handover. This situation is also considered here to be a semi-soft handover situation for the downlink channel.